The present invention relates to an electronic endoscope provided with a solid-state image sensor disposed at the distal end thereof.
Recently, there have been provided electronic endoscopes called as an electronic scope with a solid-state image sensor such as a CCD(charge coupled device) disposed at the distal end thereof for picking up an image of a portion being observed and displaying the image on a screen of a monitor television. These electronic endoscopes have a picture processor and a drive circuit of a solid-state image sensor which are disposed at a position several meters away from the solid-state image sensor. In this structure, a drive signal to be applied to the solid-state image sensor from the drive circuit fails to obtain an accurate representation of a signal waveform at the input terminal of the image sensor because of a long signal transmitting line, so that a good picture can not be obtained. To eliminate this disadvantage, the applicant of the present invention has proposed a solution in Japanese Application Sho No. 60-225368. This application discloses that a plurality of impedance matching circuits composed of a resistor and a capacitor are provided within a light source unit including a drive circuit and the like and one of the plurality of matching circuits is selected according to an electronic endoscope in use so that a drive signal can be an accurate and optimum waveform at an input terminal of the solid-state image sensor.
In addition, another Japanese Laid Open Application Sho No. 60-250608 discloses an electronic endoscope in which circuit parts required to properly operate a solid-state image sensor disposed at the distal end thereof are arranged in the middle of the insertable portion thereof by providing a second hard portion.
In the former application Sho No. 60-225368, a drive signal may or may not obtain an optimum waveform at an input terminal of a solid-state image sensor because of a long signal transmitting line. As a result, in electronic endoscopes with the same cable length, when some of them are combined with a light source unit an accurate square waveform may be produced, whereas when others are combined with the light source unit overshooting, for example, may be caused, thus operations of electronic endosoopes are disadvantageously unstable. In the latter laid-open application Sho No. 61-250608, since the second hard portion is disposed in the middle of the insertable portion of an electronic endoscope, when an electronic endoscope is for use in medical treatment, for example, it greatly inflicts pain upon a patient when the endoscope is inserted into his coelom.